Animal (2013), 7:s2, pp 220–234 & Food and Agriculture Organization of the United Nations 2013 animal doi:10.1017/S1751731113000876 Technical options for the mitigation of direct methane and nitrous oxide emissions from livestock: a review - P. J. Gerber1 , A. N. Hristov2, B. Henderson1, H. Makkar1,J.Oh2, C. Lee2, R. Meinen2, F. Montes3,T.Ott2, J. Firkins4, A. Rotz5, C. Dell5, A. T. Adesogan6,W.Z.Yang7, J. M. Tricarico8, E. Kebreab9, G. Waghorn10, J. Dijkstra11 and S. Oosting11 1Agriculture and Consumer protection Department, Food and Agriculture Organization of the United Nations, Vialle delle terme di Caracalla, 00153 Rome, Italy; 2Department of Animal Science, The Pennsylvania State University, University Park, PA 16802, USA; 3Plant Science Department, The Pennsylvania State University, University Park, PA 16802, USA; 4Department of Animal Sciences, The Ohio State University, Columbus OH 43210, USA; 5Department of Animal Sciences, USDA-Agricultural Research Service, Pasture Systems and Watershed Management Research Unit, University Park, PA 16802, USA; 6University of Florida, Gainesville, FL 32608, USA; 7Agriculture and Agri-Food Canada, Lethbridge AB, Canada T1J 4B1; 8Innovation Center for U.S. Dairy, Rosemont, IL 60018, USA; 9Department of Animal Sciences, University of California, Davis, CA 95616, USA; 10DairyNZ, Hamilton 3240, New Zealand; 11Department of Animal Sciences, Wageningen University, 6700 AH Wageningen, The Netherlands (Received 28 February 2013; Accepted 15 April 2013) Although livestock production accounts for a sizeable share of global greenhouse gas emissions, numerous technical options have been identified to mitigate these emissions. In this review, a subset of these options, which have proven to be effective, are discussed. These include measures to reduce CH4 emissions from enteric fermentation by ruminants, the largest single emission source from the global livestock sector, and for reducing CH4 and N2O emissions from manure. A unique feature of this review is the high level of attention given to interactions between mitigation options and productivity. Among the feed supplement options for lowering enteric emissions, dietary lipids, nitrates and ionophores are identified as the most effective. Forage quality, feed processing and precision feeding have the best prospects among the various available feed and feed management measures. With regard to manure, dietary measures that reduce the amount of N excreted (e.g. better matching of dietary protein to animal needs), shift N excretion from urine to faeces (e.g. tannin inclusion at low levels) and reduce the amount of fermentable organic matter excreted are recommended. Among the many ‘end-of-pipe’ measures available for manure management, approaches that capture and/or process CH4 emissions during storage (e.g. anaerobic digestion, biofiltration, composting), as well as subsurface injection of manure, are among the most encouraging options flagged in this section of the review. The importance of a multiple gas perspective is critical when assessing mitigation potentials, because most of the options reviewed show strong interactions among sources of greenhouse gas (GHG) emissions. The paper reviews current knowledge on potential pollution swapping, whereby the reduction of one GHG or emission source leads to unintended increases in another. Keywords: greenhouse gases, climate change, animal production, animal feeding, manure management Implications Introduction The paper reports on technical options for the mitigation of In view of livestock’s sizeable contribution to global warming, livestock sector’s contribution to climate change. On the this review assesses the veracity, efficacy and feasibility of basis of a comprehensive review of in vivo studies, it pro- the many mitigation options that have been put forward by vides the researcher and the livestock sector stakeholder practitioners and researchers over the past few decades. This with concise information on exiting mitigation practices, review spans the breadth of the literature on mitigation, their effectiveness and interactions. Uncertainties and areas drawing primarily on a recent comprehensive review of miti- for further research are also highlighted. It is hoped that the gation measures for livestock by Hristov et al. (2013), which paper will contribute to the identification of lower green- incorporates information from over 900 references. This review house gas -emission pathways for livestock production. also benefitted from an expert consultation, which assembled leading global scientists to peer-review and improve the - E-mail: [email protected] review by Hristov et al. (2013). Much of the discussion on 220 Mitigation of GHG emissions from livestock interactions between mitigation practices and greenhouse The emission intensity (Ei) of a commodity, measured as gases (GHGs) in this paper is derived from the workshop. the quantity of GHG emissions generated per unit of output, Livestock production plays a crucial role in food security, is a useful metric for several reasons. It allows for mean- rural livelihoods and development at large (Herrero et al., ingful comparison of emissions especially within, but also 2013). It also accounts for a substantial share of global between, commodities. It is also very closely linked to the anthropogenic GHG. If all emissions along the livestock productivity of the system, measured in terms of output per supply chain are considered, this contribution amounts to animal, or on a whole herd basis. Moreover, as productivity 7.1 Gt CO2-eq, for the 2005 reference period (FAO, 2013a improvements can increase profits at the same time as and 2013b). When considering only the direct CH4 and N2O lowering Ei, they may also present opportunities to profitably emissions from enteric fermentation and manure (including invest in mitigation. The Ei metric can also accommodate its application), livestock are estimated to contribute 5.4 Gt emission reductions (or emissions stabilization) alongside CO2-eq to global emissions (FAO, 2013a and 2013b). expanding output, which is important, given that livestock Large differences in emission intensities and/or quantities commodity production is projected to grow at a steady pace are observed between species, regions and production sys- until at least the middle of this century. Mitigation measures tems. When considering total supply chain emissions, cattle that improve productivity also have the best prospects for (beef and dairy) production generates 4.6 Gt, the largest minimizing the trade-offs between mitigation, food security share of global livestock emissions by some margin. This figure and producer welfare. At the same time, profitable pro- drops to a still significant 3.3 Gt when only the direct CH4 and ductivity improvements will, in many cases, encourage the N2O emissions from enteric fermentation and manure are sector to expand; therefore, from a policy perspective they considered (FAO, 2013b). This massive contribution stems from are necessary options, which can only be sufficient for cattle’s dominant global share of live animal biomass and, like mitigation if coupled with policies to restrict the sector’s all ruminant animals, from their fermentative digestive system. total quantity of emissions. Other livestock species have much lower and similar levels This review focuses on mitigation options for direct emis- of emissions, even when considering the full lifecycle of sions: enteric CH4 mitigation practices for ruminant animals emissions: pigs (0.7 Gt CO2-eq), poultry (0.7 Gt CO2-eq), (only in vivo studies were considered in the original review by buffalo (0.6 Gt CO2-eq) and small ruminants (0.5 Gt CO2-eq) Hristov et al., 2013) and manure mitigation practices for both (FAO, 2013a and 2013b). ruminant and monogastric species. Mitigation options that Of the 3.3 Gt of direct cattle GHG emissions, CH4 from reduce Ei only by increasing herd productivity (e.g. animal enteric fermentation is the largest source, accounting for a husbandry, genetics and health management) while keeping 71% share. Manure N2O, particularly from deposition on herd GHG output constant (or increasing it proportionally pasture, accounts for the next largest share (25%), whereas less than productivity) are not included in this review, despite the remaining 4% is from manure CH4 (FAO, 2013b). their great relevance among low-intensity ruminant systems Direct emissions typically account for 15% and 35% in (Gerber et al., 2011; FAO, 2013a and 2013b). poultry and pig production, respectively. Emissions related to In the following section, mitigation options for reducing manure storage and processing are important for pig supply enteric CH4 production are reviewed. These options fall into chains with 27% of emissions (FAO, 2013a). two broad categories of feed supplements and feeds/feeding In addition to direct emissions, livestock supply chains management. Following this, mitigation options for manure release GHG through animal feed production and post-harvest management are reviewed. These include dietary manage- activities. Feed production is the main source of indirect emis- ment options, but the focus is mainly on a range of ‘end-of- sions and is particularly important for the monogastric sector. pipe’ options for the storage, handling and application phases Emissions (primarily N2O) from feed production are almost of manure management. equal in size to direct emissions. They represent 36% of cattle After this, the role of interactions between mitigation